Researchers are examining recordings made at the ocean’s bottom in the acoustics lab at Lamont-Doherty Earth Observatory in Palisades, New York, a campus perched on a wooded cliff above the Hudson River where the buildings resemble a quiet liberal arts college rather than a location producing some of the most important earth science on the planet. Those who are not familiar with deep-sea acoustics will not anticipate the sounds.
Silence does not exist. The ocean floor is not silent. It was never the case. The calls of whales, the grinding of tectonic plates, the roar of underwater storms, and, increasingly, a persistent low-frequency hum that didn’t exist at this intensity fifty years ago make up this layered acoustic environment. This is the sound of human industry, conducted millions of times a day across every major shipping lane on Earth, traveling downward through the water column and spreading across the seafloor like a fog that never clears.
Researchers studying ocean acoustics distinguish between “biophony” and “anthrophony”—biological sound and noise produced by humans—for more reasons than merely semantics. It represents an actual, quantifiable shift in the sound of the waves. The ocean is a very effective acoustic medium; sound moves through it around 4.3 times quicker than it does through the air. Under the correct circumstances, hydrophones on the opposite side of the ocean can pick up a whale’s song generated in the North Atlantic.
Continuously operating through the same water, commercial shipping engines generate noise at frequencies that coincide with dolphins’ echolocation clicks, baleen whales’ communication calls, and fish’s acoustic signals for finding mates and prey. Animals that rely on sound to locate, hunt, and socialize are effectively going deaf in a noisy room when the anthropogenic layer climbs above the biological layer. Meanwhile, humans install more speakers and wonder why the inhabitants appear confused.
The use of acoustic monitoring for ocean acidification is more subtle and sophisticated technically. The ocean’s chemistry changes when it takes in carbon dioxide from the atmosphere; it gets more acidic, which has an impact on how sound travels through it. In particular, low-frequency noises travel farther in acidified water than they would have in the ocean a century ago because a more acidic ocean absorbs some sound frequencies less effectively.
By analyzing the distance and clarity of sound traveling through a particular water column, researchers are purposefully utilizing this relationship to deduce changes in the chemistry of seawater. By using the ocean’s acoustic characteristics as a stand-in for its chemical state, this remote sensing method enables monitoring at distances where physical sampling is not feasible.
The research yields its most shocking individual findings in the seismic detection work. Because the signal diffused before reaching detectors on shore, hydrophone arrays moored on the seafloor and towed by research vessels have captured tectonic events—plates shifting and fracturing beneath the ocean—that were invisible in conventional land-based seismic networks.
Researchers can triangulate the position and nature of a seismic source that would otherwise go unnoticed by using the ocean as a transmission medium to transport the auditory signature of a fracture event hundreds of kilometers. Coastal planners and tsunami warning systems rely on hazard maps, and some of those sources have turned out to be in previously unknown seismically active places.
It seems as though the ocean’s auditory ecology is evolving in ways that will take decades to completely characterize, as researchers detail what the hydrophone arrays are recording year after year. The baseline for noise is increasing. In reaction, the biological signals that coexist with it are changing. For the first time, the geological signals can be heard well enough to cause surprises.
It’s currently unknown how much of the behavioral shift in marine mammals may be attributed to noise as opposed to other stressors, or how the relationship between acidification and acoustics will vary as carbon concentrations rise. However, the recordings are continuing, the data is mounting, and what Lamont-Doherty is extracting from the deep ocean below is a record of a changing world that most people above it are unaware is being written.
